Study species and site

Ajuga decumbens is an annual or biennial herb endemic to China. The plant is ~10–30 cm in height and is widely distributed in the south area of the Yangtze River. It inhabits streamsides, roadsides and wet areas. Individual plants produce multiple verticillasters (12.17 ± 0.42, n = 60) that arise from the grass roots, and each inflorescence has 5–12 crowded whorls bearing 2–16 white flowers (Fig. 1). Flowers are zygomorphic and nectar-rich. The calyx consists of five nearly equal lobes with densely pilose, while the corolla tube is nearly twice as long as the calyx with sparsely pilose. The corolla has a very short two-lobed upper lip and a three-lobed lower lip (Fig. 1). Each flower produces four epipetalous stamens arranged in two rows, four ovules and a bifid stigma (Wu and Li 1977; Fig. 1). The anthers dehisce longitudinally. The flowering season of this species in China is usually from March to June.

Floral morphology and inflorescence trait of Ajuga decumbens. (A) to (D) indicate male phase, middle phase, female phase and witling phase, respectively, while (E) shows the inflorescence trait. Scale bar = 1 cm.

All experiments were conducted in a natural population of A. decumbens at Wuhan Botanical Garden (30.551°N, 114.429°E; 23 m a.s.l.), Hebei province, China. Before flower anthesis, 100 plants from the natural population were randomly selected and transplanted into pots on 15 February 2013. Each individual was planted in a 25-cm pot filled with the original surface soil from the natural population area. All the plants were managed under the same conditions, for example, with the same soil and water source. They were initially bagged with nylon net mesh to keep them away from any kind of insects by the side of the natural population. The transplanted plants were watered once every 3 days during our investigations. The transplanted plants were selected for further investigations if they showed similar performance as those of natural population in plant height, number of inflorescences per plant and number of flowers per inflorescence.